Ki is the Japanese word for a concept that has been variously translated as mind, spirit, energy, life force, or intention. Although it is generally associated with Asian cultures (in Chinese it is Ch'i; in Korean it is Qi) almost everyone has experienced what they considered a "sixth sense" or a "gut feeling" at one time or another.
Have you ever:
If you answered "yes" to any of these, then you have some experience with ki.

In combat, we look to attack when our opponent is weak-minded or unready (called kyo). To find their kyo, or to sense their intention to strike, we benefit if we read not only our opponent's physical movements, but also their ki. Mr. Sugiyama sometimes calls this "developing radar," since the ability to read another persons' ki gives you the same type of advance warning.
Given the broad range of translations of the word, it should be clear that the idea of ki has many other applications as well. The shouts that karate students give are called kiai, meaning a focusing of ki, and the contraction of the stomach helps to generate power while executing techniques. Exercises that claim to harness ki have been proven to improve circulation and health. Many people believe that the ability to control ki can even help to heal injuries.